1. Field of the Invention
The present invention relates to a preflux having an improved operability and performance and comprised mainly of a 2-alkylbenzimidazole derivative having a good heat resistance, more particularly to a preflux suitable for surface treatment of copper or copper alloy patterns on a hard printed circuit board or flexible printed circuit board.
2. Description of Related Art
The art of this type has been proposed by, for example, JP 56-18077B and JP 4-72072A.
JP 56-18077B discloses a preflux comprised mainly of an imidazole derivative and further containing a chelating agent reacting with copper ions. Treatment of a printed circuit board with the preflux allows copper ions to release from the circuit board and react with the chelating agent to form a copper complex which is then removed out. Therefore, an anticorrosive film made of imidazole compounds can be formed only on copper or copper alloy patterns in printed circuit boards having printed contacts made of gold, platinum, silver, tin or rhodium and thus no masking is required on the printed contacts. The prior art forms a very thin, for example, 0.1-0.5 .mu.m thick film of an imidazole compound on an exposed area of the copper or copper alloy pattern on the printed circuit board. A main one of prior art electronic part-joining methods has been a pin or lead insertion mounting method. The film of the imidazole compound preflux functions as an anticorrosive film for copper or copper alloy patterns until the patterns are subjected to flow soldering. The preflux film is dissolved by a postflux which is used in pretreatment for the flow soldering to expose a clean copper or copper alloy area which is susceptible to the flow soldering.
However, in recent years, a surface mounting method has often been used as methods of joining electronic parts to printed circuit boards. Therefore, the printed circuit boards are repeatedly exposed to high temperatures in pre-setting of chip parts, soldering of surface mounted parts and flow soldering of inserted parts and chip parts. This causes the preflux film of the imidazole compound to be degraded by heat in pre-setting chip parts or soldering surface mounted parts. Therefore, in the subsequent flow soldering, the dissolution of the preflux film with the postflux is reduced, so that a residual amount of the preflux film disadvantageously prevents the solderability of the copper or copper alloy patterns.
In the prior art method above, reduction of the imidazole derivative is easily effected at a concentration of copper ions released from printed circuit boards in the preflux solution of 10 ppm or higher. Thus, the anticorrosive film of the imidazole compound is formed also on the surfaces of printed contacts made of gold, platinum, silver, tin or rhodium, so that the function of the printed contacts is lost.
However, there is a small amount of increase in the contact resistance of the printed contact made of gold even at a copper ion concentration of less than 10 ppm. From this it can be presumed that a minor amount of the imidazole compound is precipitated on the surface of the printed contact. In recent years, the highdensity packaging on printed circuit boards has been required and accordingly formation of multileads and fining of lead pitches have been developed. Thus, there is a tendency that used amounts of parts to be surface mounted, such as a tape automated bonding (TAB), chip on board (COB) or flip chip, is increasing. When such parts are to be joined to the copper pattern on printed circuit board, the pattern is necessary to be plated with gold, silver, aluminum, tin or a solder. Since the joint surface area between the copper pattern and the leads of the parts is very small, the surface of the copper pattern to be plated is required to be highly clean in order to secure the reliability of lead joint. Where the copper ion concentration is less than 10 ppm, a minor amount of imidazole compounds precipitated has an adverse influence on the reliability of lead joint.
JP 4-72072A discloses a preflux comprising an aqueous solution containing a 2-alkylbenzimidazole derivative, organic acid and zinc or a zinc compound. Films of the 2-alkylbenzimidazole derivatives are disclosed by, for example, JP 46-17046B, JP 49-26183B and JP 61-41988B. The derivatives are thermally more stable than films of 2-long-chain-alkylbenzimidazoles and have adequate heat resistance for the method of surface mounting on printed circuit boards.
Furthermore, addition of zinc or a zinc compound can further improve the heat resistance. In the prior art, the 2-alkylbenzimidazole derivatives in the preflux are reduced by reaction with copper or copper alloy patterns in printed circuit boards and thus the films of the 2-alkylbenzimidazole derivatives are formed only on the copper or copper alloy patterns, while the surfaces of the patterns are oxidized, so that the copper is dissolved in copper ions into the preflux. On the other hand, when the concentration of the dissolved copper ions exceeds 10 ppm in the preflux, the reduction of the 2-alkylbenzimidazole derivatives becomes remarkably easy and the films of the 2-alkylbenzimidazole derivatives may also be formed on the printed contacts made of gold, platinum, silver, tin, rhodium or the like, or on the joint land of the surface mounted parts, made of gold, silver, aluminum, tin, a solder or the like, thereby disadvantageously reducing the reliability of lead joint.
In recent years, more leads have been given to an electronic part to be surface mounted and lead pitches have been made finer for high density packaging. Thus, this provides a tendency that the employed amounts of the parts to be surface mounted, such as tape automated bonding (TAB), chip on board (COB), flip chips and the like, increase. The joint land on printed circuit board of such parts. Such parts to be surface mounted are plated with gold, silver, aluminum, tin, a solder or the like. Surface treatment of the partially plated copper patterns at the leads thereof with gold, silver, aluminum, tin or a solder will increasingly be important in future.
Furthermore, a solution of the prior art preflux contains copper ions released from the copper pattern of printed circuit board accumulated therein. As the concentration of copper ions in the preflux solution increases, the speed at which the preflux film is precipitated on the copper pattern of printed circuit board, also increases. Therefore, there is a tendency that as the number of the printed circuit boards treated increases, the preflux film becomes thicker within a given treatment time. In recent years, surface-mounted printed circuit boards are repeatedly subjected to various soldering steps at high temperatures, such as soldering of surface mounting parts, pre-setting of chip parts, flow soldering of insert or chip parts. Therefore, in the case where the preflux film gets excessively thick, the preflux film is inadequately dissolved when brought into contact with a postflux at the flow-soldering stage, so that the solderability of the copper pattern is degraded. Moreover, soldering without cleaning, in which no cleaning of printed circuit boards with a solvent is conducted after the soldering under regulation of solvents, employs a postflux having a low activity. Therefore, in the case where the preflux film gets excessively thick, the solderability of the copper pattern is degraded. From this it is seen to be important that the preflux film is stably precipitated and thickness of the preflux film is controlled.